docs: restructure Kallithea upgrade instructions This creates a separate "Upgrading Kallithea" section, instead of trying (and failing) to repeat the same information for each installation page. Trying to maintain upgrade instructions for each installation method was perhaps overly ambitious. In practice, only the Linux instructions were up-to-date. (Incidentally, this is kind of a throwback to Kallithea 0.1, which also had a separate upgrade section.)